Radiation-sensitive compositions are routinely used in the preparation of imageable materials including lithographic printing plate precursors. Such compositions generally include a radiation-sensitive component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties, imaging performance, and image characteristics.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged and/or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and unexposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the unexposed regions become an image.
Various negative-working radiation compositions and imageable elements are described in and U.S. Pat. Nos. 6,309,792 (Hauck et al.), 6,569,603 (Furukawa), 6,893,797 (Munnelly et al.), 6,787,281 (Tao et al.), and 6,899,994 (Huang et al.), U.S. Patent Application Publications 2003/0118939 (West et al.), 2005/0008971 (Mitsumoto et al.), and 2005/0204943 (Makino et al.), and EP 1,079,276A (Lifka et al.), EP 1,182,033A (Fujimaki et al.), and EP 1,449,650A (Goto).
Independently of the type of imageable element, lithography has generally been carried out using a polymeric or metal substrate (or “support”) such as a substrate comprising an aluminum or aluminum alloy support of various metallic compositions. The surface of the metal sheet is generally roughened by surface graining in order to ensure good adhesion to a layer, usually an imageable layer, that is disposed thereon and to improve water retention in non-imaged regions during printing. Various aluminum support materials and methods of preparing them are described in U.S. Pat. Nos. 5,076,899 (Sakaki et al.) and 5,518,589 (Matsura et al.).
For example, to prepare aluminum-containing substrates for lithographic elements, a continuous web of raw aluminum can be treated, for example, using a sequence of known steps. Such steps may include taking the continuous aluminum web through a degreasing section of a manufacturing machine to remove oils and debris from the aluminum web, an alkali etching section, first rinsing section, graining section (that can include mechanical or electrochemical graining, or both), second rinsing section, post-graining acidic- or alkali-etching section, third rinsing section, anodization section using a suitable acid to provide an anodic oxide coating, fourth rinsing section, post-treatment section, final or fifth rinsing section, and drying section, before either being rewound or passed on to coating stations for application of imageable layer formulations.
In the anodization section, the aluminum web is treated to form an aluminum oxide layer on its surface so it will exhibit a high degree of mechanical abrasion resistance necessary during the printing process. This oxide layer is already hydrophilic to some degree, which is significant for having a high affinity for water and for repelling printing ink. However, the oxide layer is so reactive that it can interact with components of the imageable layer in the imageable element. The oxide layer may partially or completely cover the aluminum substrate surface.
In the post-treatment section, the oxide layer is covered with a hydrophilic protective layer (also known in the art as a “seal”, “sublayer”, or “interlayer”) to increase its hydrophilicity before one or more imageable layer formulations are applied. The hydrophilic protective layer can be applied by immersing the web in the post-treatment solution or by spraying the solution onto the web (with optional recovery tank, filter, and fluid delivery system). A suitable interlayer may also ensure that during development, the soluble regions of the imageable layer are easily removed from the substrate, leaving no residue and providing clean hydrophilic backgrounds. The hydrophilic interlayer can also protect the aluminum oxide layer against corrosion during development with highly alkaline developers and from dye penetration from the imageable layer.
Other hydrophilic interlayers are prepared from formulations including poly(vinyl phosphonic acid) (PVPA), vinyl phosphonic acid/acrylic acid (VPA/AA) copolymers, and poly(acrylic acid) (PAA) as described for example in U.S. Pat. No. 4,153,461 (Berghauser et al.) and EP 0 537 633B1 (Elsaesser et al.). U.S. Pat. No. 6,218,075 (Kimura et al.) describes the treatment of metal substrates with various compositions of poly(vinyl phosphonic acid).
U.S. Pat. No. 4,427,765 (Mohr et al.) describes the use of a water-soluble organic polymer having acidic functional groups (such as phosphorous or sulfonic acid groups) with a salt of a divalent metal cation. Various organic polymers and metal cations are described that are believed to form a complex of polymer and metal cation. U.S. Pat. No. 5,314,787 (Elsaesser et al.) describes the treatment of aluminum substrates with a hydrophilic polymer solution followed by treatment with a solution containing divalent or polyvalent metal cations.
In addition, U.S. Pat. No. 7,214,468 (Takahashi et al.) describes the use of flexible supports having a hydrophilic layer comprised of metal oxide sols. Reactive functional groups have also been added to aluminum and polymer supports as described in U.S. Pat. No. 5,807,659 (Nishimiya et al.) and EP Publication 1,495,866 A1 (Mitsumoto et al.). An intermediate layer comprising a polymer derived from a phosphate-substituted methacrylate is described in EP Publication 1,788,429 (Loccufier et al.).
A hydrophilic layer on the metal support of lithographic elements described in EP Publication 1,791,699 (Fiebag et al.) contains a phosphono-substituted siloxane. EP Publication 1,787,166 (Strehmel et al.) describes the use of phosphate-containing compounds in interlayers.
U.S. Pat. No. 7,049,048 (Hunter et al.) describes an interlayer material on an aluminum support in lithographic elements that includes a copolymer having acidic groups and silyl groups that are substituted with three alkoxy or phenoxy groups.